1. Field of the Invention
This invention relates to a feed speed control method for a numerical control device, and more particularly, to a feed speed control method for a numerical control device by which a correct feed speed is automatically derived.
2. Background of the Related Art
In numerical control devices (CNC), a workpiece is machined to a desired shape by moving a tool at a commanded speed along a machining path instructed by a machining program.
To efficiently effect the machining operation at a high precision, the machining speed for each block must be determined by taking into account the shape to be machined, the maximum torque of a motor, the degree of shock to a machine, and the like. In particular, when a machining program is prepared, an exact stop command is inserted at, for example, a corner portion of an instructed path for a workpiece at which the cutting direction is greatly changed.
To create such a program, however, a relatively high level of skill, and a great deal of labor are required. Namely, a programmer must determine an optimum machining speed (feed speed) by taking the machine tool, the shape of a workpiece, and the like into consideration.
Further, if the machining speed is set to a value which is unnecessarily low in comparison with the high performance machining speed of the machine tool, when the program is prepared, a problem arises in that the performance of the machine tool cannot be fully utilized.
The inventors of this invention proposed, in Japanese Patent Application No. 63-224932, a method of deriving a feed speed which causes a speed variation of each axis at a corner portion to be smaller than a maximum permissible speed difference derived from a predetermined maximum permissible torque, for the control of the speed.
Further, as a related application, a method of controlling the acceleration/deceleration was proposed in Japanese Patent Application No. 1-15595, which method can reduce a load caused by a repetition of an acceleration/deceleration of a servomotor when blocks continue for infinitesimal distribution distances.
The method proposed in Japanese Patent Application No. 63-224932 is effective for a corner portion with a large angular variation, but a correct feed speed cannot be derived where individual speed variations are small but an actual acceleration for each axis is large, because the variations occur successively in a short time; for example, in the case of a free curve approximately represented by straight lines. In this case, the mechanical shock may be disadvantageously increased and an excessively large load may be applied to a servomotor.